1. Field of the Invention
The present invention relates to an illumination device for illuminating a non-self light emission display element and a display device which is provided with the illumination device and used for an electronic device. In particular, the present invention relates to a liquid crystal display device used for a portable information device, a mobile telephone, and the like and an illumination device such as a frontlight unit or a backlight unit to illuminate the display element.
2. Description of the Related Art
In recent years, most of display devices used for a mobile telephone, a mobile computer, and the like are liquid crystal display devices capable of obtaining a high-definition color image with reduced power consumption. A liquid crystal element used for the liquid crystal display devices is a non-self light emission type, so the liquid crystal element is illuminated by an illuminating device using a high-intensity white LED as a light source.
In particular, a reflection type liquid crystal display device which has a large aperture and is bright or a liquid crystal display device of a double side visible type which can display image information on both front and rear screens is used for the mobile telephone. The white LED used to illuminate a display element of each of the liquid crystal display devices has a structure in which a resin into which a yellow phosphor is dispersed is provided immediately in front of a light emitting surface of a blue LED made of InGaN, GaN, or the like. According to the structure, yellow light can be mixed with original blue light to obtain white light. An yttrium aluminum garnet (YAG) phosphor in which YAG is doped with a rare-earth element has been widely known as a phosphor for converting the blue light into the yellow light. A method of producing white light by additive mixing of blue, red, and green using a mixture of red and green light emitting phosphors instead of the YAG phosphor has been known (see, for example, JP 10-163535 A). A chalcogenide phosphor or a nitride phosphor which is doped with a rare-earth element has been widely known as a phosphor for converting the blue light into green light or red light with relatively high efficiency. There has been disclosed an LED display device in which a plurality of light emitting elements, each of which emits light having a wavelength equal to or shorter than the wavelength of the blue light, are arranged on a printed circuit board which has an arbitrary shape and an arbitrary area and includes circuits formed therein and the respective light emitting elements are coated with a translucent resin containing a wavelength conversion material (see, for example, JP 11-121802 A).
However, in the case of additive mixing of two colors using the blue LED and the YAG phosphor (i.e., pseudo white LED), the amount of light component having a wavelength region of 600 nm or more is small. Therefore, an LCD module having high color reproducibility cannot be realized. In general, when the pseudo white LED is used as a light source, it is very difficult for a current color filter to exceed an NTSC ratio of 100%. As described in JP 10-163535 A, when a structure in which two kinds of phosphors for converting blue light into green light and red light based on blue excitation are dropped on the light emitting surface of the blue LED is used, additive mixing of three colors (i.e., three-wavelength white LED) is possible, so the LCD module having high color reproducibility can be realized. However, the chalcogenide phosphor doped with the rare-earth element causes a chemical reaction with a reflective film included in the LED, so a problem in which a reflection characteristic deteriorates occurs in many cases. When the white light is to be produced based on blue excitation using the mixture of the green phosphor and the red phosphor, a nitride phosphor or a chalcogenide phosphor may be used as the red phosphor. An excitation wavelength of each of the phosphors includes that of the green light emitted from the green phosphor, so there is a problem in that intensity efficiency is low.